1. Field of the Invention
The present invention relates to a back irradiation type light-receiving device, such as back irradiation type charge coupled device (CCD) or back irradiation type active pixel sensor (APS), which is applicable to radiation of energy rays yielding a large absorption coefficient such as ultraviolet rays, .gamma.-rays, and charged particle rays; and a method of making such a back irradiation type light-receiving device.
2. Related Background Art
In general, practicable CCD imaging devices typically employ any of three systems, i.e., frame transfer (FT), full frame transfer (FFT), and interline transfer (IT). Among others, the FFT system is mainly used for measurement.
The FFT system is advantageous in that, since there is no accumulating section, its light-receiving section can have a larger area, thereby attaining a high light utilization factor, thus making it suitable for measuring weak light. On the other hand, since incident light can be absorbed by a charge transfer electrode therein, the sensitivity of the system remarkably decreases with respect to an input with a large absorption coefficient such as light having a short wavelength.
The light-receiving section of a typical FFT type CCD is configured such that a plurality of polysilicon electrodes cover the surface of the light-receiving section without any clearance, while a PSG film having a thickness of several micrometers is overlaid thereon in order to separate the individual electrodes from each other. In such a configuration, since polysilicon absorbs electrons and light having a wavelength not longer than 400 nm, light with a short wavelength cannot reach the light-receiving section, thus failing to contribute to photoelectric conversion.
Known as such a photodetector is that having a substrate with a thin light-receiving section on the order of 15 to 20 .mu.m, while light is irradiated from the back side of the device-forming surface. The photoelectric conversion section is formed under a gate oxide film and is covered with polysilicon electrodes without any clearance, thus absorbing light having a short wavelength incident thereon from its front side. On the back side of the substrate, since there is no obstacle other than the thin oxide film, a high sensitivity is expected with respect to light having a short wavelength incident thereon from the back side.
This back irradiation type CCD is sensitive to light having a wavelength as short as about 200 nm and can further be applied to an electron bombardment type CCD imaging device. Since this device can utilize the multiplying action of a signal charge generated upon electron bombardment, it is expected to become a highly sensitive imaging device.
The necessity for thinning a back irradiation type light-receiving device such as back irradiation type CCD lies in the following points.
In the back irradiation type light-receiving device, as mentioned above, the back side opposite to the front side of the substrate provided with a charge-reading section or the like serves as an entrance surface for light. On the other hand, light with a wavelength of 200 to 300 nm (ultraviolet light) exhibiting a large absorption coefficient is substantially absorbed at a position slightly inside the entrance surface. Specifically, in the case of a silicon substrate, incident light is substantially absorbed thereby within a depth of 0.01 .mu.m from the entrance surface.
Accordingly, photoelectrons generated in the vicinity of the back side are substantially lost by recombination before they diffuse into a potential well on the front side. Even in the case where the photoelectrons reach the potential well, during the time when they diffuse into a long path of several hundred micrometers, signals may mingle with each other, thus remarkably deteriorating resolution.
Also, in the back irradiation type light-receiving device, in addition to thinning the light-receiving section, it is necessary for the backside entrance surface to be provided with a layer known as accumulation layer, so as to form a potential slope. FIG. 3 is a view for explaining accumulation. In FIG. 3, the right side and left side of the drawing indicate the front side and the back side, respectively. Formed by growth on the back side of a silicon substrate 910 is an oxide film 952 which serves as a protective film.
The oxide film 952, however, always have an oxide film charge and an interface level, each of which functions to deplete the back side of the substrate 910. Namely, in view of a potential profile, as indicated by solid line in FIG. 3, the potential with respect to an electron decreases as the position is nearer to the oxide film 952 on the back side. That is, a photoelectron generated at a position close to the back side fails to reach the potential well of the CCD, and is pushed to the interface between the backside oxide film 952 and silicon so as to be destined for recombination. Accordingly, after the light-receiving section is thinned and its back side is oxidized, the substrate 910 near the backside oxide film 952 is set to an accumulation state so as to attain a potential profile indicated by dotted line in FIG. 3. As a result, even photoelectrons generated at a position close to the back side can efficiently reach the potential well of the CCD on the front side.
The accumulation layer is formed by a method comprising the steps of ion-injecting boron into the backside oxide film 952, and heat-treating it at a temperature not lower than 800.degree. C. so as to activate the injected atoms.
U.S. Pat. No. 4,923,825 discloses a technique (hereinafter referred to as conventional example) concerning a method of making a back irradiation type light-receiving device, which enables a heat treatment such as high-temperature annealing after the formation of a grown oxide film and the ion injection therein, like that mentioned above, and a back irradiation type light-receiving device manufactured by this method.
FIG. 4 is a configurational view showing a back irradiation type light-receiving device using the technique of the conventional example. As shown in FIG. 4, this device comprises: (a) a semiconductor thin plate 910 mainly composed of silicon, in which a charge coupled device (CCD), as a charge-reading section 911, is formed on its front side 916; (b) a field oxide film 920 formed around the charge-reading section 911; (c) polysilicon leads 931 formed on the charge-reading section 911 and at peripheral portions of the charge-reading section 911 on a front side 926 of the field oxide film 920; (d) polysilicon electrodes 932 formed on regions separated by the field oxide film 920 from the region where the charge-reading section 911 is formed; (e) metal leads 933 electrically connecting the polysilicon leads 931 with their corresponding polysilicon electrodes 932; (f) a reinforcement member 940 made of borosilicate glass deposited on the front side 916 of the semiconductor thin plate 910 and on the front side 926 of the field oxide film 920; (g) an accumulation layer 951 formed on a back side 917 of the semiconductor thin plate 910; (h) a protective oxide film 952 formed on the back side of the accumulation layer 951; and (i) metal electrodes 960 formed on the polysilicon electrodes 932.
This back irradiation type light-receiving device is manufactured in the following manner. FIGS. 5A to 5F are views showing, step by step, a method of making the back irradiation type light-receiving device in accordance with the conventional example.
First, on a semiconductor substrate 919, the charge-reading section 911 is formed. Then, the field oxide film 920 is formed. Thereafter, the polysilicon leads 931 and the polysilicon electrodes 932 are formed (see FIG. 5A). Subsequently, the metal leads 933 for electrically connecting the polysilicon leads 931 to their corresponding polysilicon electrodes 932 are formed (see FIG. 5B).
Next, borosilicate glass is deposited on a surface constituted by the front side 916 of the semiconductor substrate 916 and the front side 926 of the field oxide film 920, so as to form the reinforcement member 940 (see FIG. 5C). Subsequently, the semiconductor substrate 919 is thinned so as to form the semiconductor thin plate 910. After the surface of the semiconductor thin plate 910 is provided with the protective oxide film 952, it is activated by ion injection and heating, thus forming the accumulation layer 951 (see FIG. 5D).
Then, of the semiconductor thin plate 910, a portion surrounding the charge-reading section 911 is removed by etching so as to expose the polysilicon electrodes 932 (see FIG. 5E). Subsequently, the metal electrodes 960 are formed on the polysilicon electrodes 932 so as to become bonding pads (see FIG. 5F). Thereafter, the bonding pads are subjected to wire bonding, thus yielding the back irradiation type light-receiving device shown in FIG. 4.
The metal leads 933 held between the semiconductor thin plate 910 and the reinforcement member 940 are heated at a high temperature during the manufacturing the time of sintering borosilicate glass when the reinforcement member 940 is formed, in the heat treatment (800.degree. C. to 900.degree. C.) at the time when the protective oxide film is grown, and in the high-temperature annealing (800.degree. C. to 900.degree. C.) after ion injection.
Accordingly, as a material for the metal leads 933, aluminum whose melting point is about 660.degree. C. cannot be used, and high-melting metals such as molybdenum and tungsten or their silicides are used.